1. Field of the Invention
The present invention relates to bearings steels and to long life rolling bearings used in automotive vehicles, agricultural machinery, construction machinery and iron-and-steel machinery and, especially, for use in transmissions and engines.
2. Description the Related Art
Conventionally, it is known that in-steel nonmetal inclusions, especially, oxide or oxide-based inclusions cause an increase, for example, in the incidence of wire breaking in wire drawing and a deterioration of the mechanical characteristics of twisting value, fatigue, etc., of a product.
Therefore, various high clean steels with a low content of in-steel oxygen have been provided. For example, Japanese unexamined patent application publication SHO. 53-76916 discloses a high clean steel, the oxygen content of which is 50 ppm or less.
The quality of steel largely depends on the number and size of nonmetal inclusion particles. Methods of determining the number and size of nonmetal inclusion particles comprises JIS-G0555, ASTM-E45 and an image-processing inclusion inspection system disclosed in Japanese unexamined patent application publication SHO. 63-309844.
Bearing life has a close relationship with the cleanness of a material. Particularly, a rolling contact part which is made of a steel with an oxide or oxide-based nonmetal inclusion (especially Al.sub.2 O.sub.3) and which experiences repeated rolling contact stress may experience the occurrence of microcracks derived from the oxide or oxide-based nonmetal inclusion. This eventually leads to flaking originating from the microcrack so that the life of a rolling bearing which includes the rolling contact part expires.
In increasing bearing life, a selection of the content of in-steel oxygen or an index of inclusion in accordance with ASTM standards or a monitoring and evaluation of the size and manner of oxide or oxide-based inclusion particles present in a bearing steel guarantees the cleanness of a material in order to underlie an evaluation of bearing life.
This cleanness guarantee must require a separation of only the oxide or oxide-based nonmetal inclusion from the matrix of a sample and a monitoring and evaluation of the nonmetal inclusion. For this purpose, there is a prior-art method of monitoring and evaluating the oxide or oxide-based nonmetal inclusion using electron beam melting (see "Iron and Steel", pages 83-90, volume 10 of 75th year in 1989 and the Journal issued May 19, 1987 by the Japan Society for the Promotion of Science, pages 5-1 to 5-14).
This prior-art teaches that an electron beam completely melts a small amount of a sample in a button shape to raise and concentrate the inclusion the top surface of the resulting molten metal. A scanning electron microscope (SEM) or the like monitors and evaluates the configurations, sizes and number of oxide or oxide-based inclusion particles after cooling and solidification.
The prior art fails to specify the ranges of the number and sizes of oxide or oxide-based inclusion particles effective to increase the rolling contact fatigue life of a rolling bearing and the relationship between the bearing life and oxide or oxide-based inclusions for securing a sufficient life of a rolling bearing. On the other hand, only conventional items of cleanness evaluation can, with difficulty, discriminate long and short lives of rolling bearing in a situation in which the bearing steel has recently been required to be high-clean, so that selecting only the conventional items failed to increase the bearing life.
The prior-art electron beam melting method entails a problem in that, since it must completely melt a sample, the electric charge power at the time of electron beam generation is excessive. Controlling amount of melting is difficult when only a portion of the sample is melted. Additionally, an inclusion present in the sample below a determined depth cannot be selectively melted and raised to the top surface of the sample.
This problem is prominent especially in steel used for rolling contact parts. That is, it is known that a macroinclusion present at a position of maximum shearing stress defined by Hertz's theory of elasticity provides one of the main causes of flaking which causes termination of the life of the rolling bearing. The position of maximum shearing stress falls within a few millimeters depth from the rolling contact surface of a rolling contact part made of the rolling contact parts steel. Therefore, the material cleanness guarantee for long bearing life requires a selective separation of only an oxide or oxide-based nonmetal inclusion present within the few millimeters depth from the rolling contact surface and a monitoring and evaluation of the separated inclusion. However, the prior art electron beam melting method fails to melt part of a sample so that it cannot selectively separate and monitor macroinclusions present at the position of maximum shearing stress.
In addition, the solidified surface structure of a sample which has experienced button-shaped melting by an electron beam and then been solidified demonstrates a marked affect in distribution of rafts after solidification. This is particularly true when the solidification of the sample produces a dendritic structure on the top surface of the sample. In such instances dendrite arms or branches of the dendritic structure entrap the rafts therebetween in a non-uniform distribution which blocks the SEM or the like from monitoring and evaluating inclusions in the sample. The above problems in the electron beam melting method reduce the accuracy of the life guarantee for the rolling bearing.
A primary object of the present invention is to provide long-life bearing steel or rolling bearings by means of considering oxide or oxide-based nonmetal inclusion particles as an evaluation item for bearing life and controlling the size and number of the inclusion particles.
Another object of the present invention is to provide a long life bearing steel, the bearing life of which is guaranteed by an electron beam melting method of high accuracy capable of monitoring and evaluation of oxide or oxide-based nonmetal inclusions.